Today I harvested my woad and processed it to make a blue dye for dying my wool. It seemed late to harvest, but the woad was still lush and green so I decided to try it. I found a good site on the internet from a woman in the U.K. who makes and also sells the powdered dye. Aerating the solution causes the dye to oxidize. This causes the dye to form a precipitate which you can then separate from the liquid to make a powder. The powder allows for longer term storage. My dye is settling now.
As I was making the dye though, it got me wondering what dye they used for the “horizon blue” uniforms the French soldiers wore. It turns out that woad, used for dying wool in Europe for millennia, was finally usurped in the 1700s by indigo from India and other places in the far east. Indigo creates deeper and darker blues than woad. By the late 1800s synthetic dyes were becoming common. A synthetic indigo dye was launched in 1897 by a German scientist, Adolf von Baeyer. At the time, the production of natural indigo was around 19,000 tonnes, but by 1914, it was down to only 1000 tonnes having been replaced by synthetic indigo. Baeyer received the Nobel Prize in chemistry in 1905 for his work on chemical dyes.
So the “horizon blue” uniforms of the French soldiers would have been made from indigo-dyed wool, but it was undoubtedly the synthetic version.